Aircraft reciprocating engine exhaust systems are exposed to extreme temperatures, vibration, and corrosive exhaust gases during operation. Regular inspection and maintenance help identify leaks, cracks, and other defects before they develop into serious safety hazards. This article explains recommended inspection procedures, common exhaust system failures, and maintenance practices.
Exhaust System Maintenance Practices
Any exhaust system failure should be regarded as a severe hazard. Depending on the location and type of failure, an exhaust system failure can result in carbon monoxide poisoning of crew and passengers, partial or complete loss of engine power, or an aircraft fire. Cracks in components, leaking gaskets, or complete failure can cause serious problems in flight. Often, these failures can be detected before complete failure. Black soot around an exhaust gasket shows the gasket has failed. The exhaust system should be inspected very thoroughly.
Exhaust System Inspection
While the type and location of exhaust system components vary somewhat with the type of aircraft, the inspection requirements for most reciprocating engine exhaust systems are very similar. This section includes a discussion of the most common exhaust system inspection items and procedures for all reciprocating engines. Figure 1 shows the primary inspection areas of three types of exhaust systems.
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| Figure 1. Primary inspection areas of three types of exhaust systems |
When performing maintenance on exhaust systems, never use galvanized or zinc-plated tools on the exhaust system. Exhaust system parts should never be marked with a lead pencil. The lead, zinc, or galvanized mark is absorbed by the metal of the exhaust system when heated, creating a distinct change in its molecular structure. This change softens the metal in the area of the mark, causing cracks and eventual failure.
After the installation of a complete exhaust system and all pieces of engine cowl are installed and secured, the engine should be operated to allow the exhaust system to heat up to normal operating temperatures. The engine is then shut down and the cowling removed to expose the exhaust system. Each clamped connection and each exhaust port connection should be inspected for evidence of exhaust gas leakage.
An exhaust leak is indicated by a flat gray or a sooty black streak on the pipes in the area of the leak. An exhaust leak is usually the result of poor alignment of two mated exhaust system members. When a leaking exhaust connection is discovered, the clamps should be loosened and the leaking units repositioned to ensure a gas-tight fit.
After repositioning, the system nuts should be retightened enough to eliminate any looseness without exceeding the specified torque. If tightening to the specified torque does not eliminate looseness, the bolts and nuts should be replaced since they have probably stretched. After tightening to the specified torque, all nuts should be safe tied. With the cowling removed, all necessary cleaning operations can be performed. Some exhaust units are manufactured with a plain sandblast finish. Others may have a ceramic-coated finish. Ceramic-coated stacks should be cleaned by degreasing only. They should never be cleaned with sandblast or alkali cleaners.
During the inspection of an exhaust system, close attention should be given to all external surfaces of the exhaust system for cracks, dents, or missing parts. This also applies to welds, clamps, supports, support attachment lugs, bracing, slip joints, stack flanges, gaskets, and flexible couplings. Each bend should be examined, as well as areas adjacent to welds. Any dented areas or low spots in the system should be inspected for thinning and pitting due to internal erosion by combustion products or accumulated moisture. An ice pick or similar pointed instrument is useful in probing suspected areas.
The system should be disassembled as necessary to inspect internal baffles or diffusers. If a component of the exhaust system is inaccessible for a thorough visual inspection or is hidden by non-removable parts, it should be removed and checked for possible leaks. This can often be accomplished best by plugging the openings of the component, applying a suitable internal pressure (approximately 2 psi), and submerging it in water. Any leaks cause bubbles that can readily be detected. The procedures required for an installation inspection are also performed during most regular inspections. Daily inspection of the exhaust system usually consists of checking the exposed exhaust system for cracks, scaling, excessive leakage, and loose clamps.
Muffler and Heat Exchanger Failures
Approximately half of all muffler and heat exchanger failures can be traced to cracks or ruptures in the heat exchanger surfaces used for cabin and carburetor heat sources. Failures in the heat exchanger surface (usually in the outer wall) allow exhaust gases to escape directly into the cabin heat system. These failures, in most cases, are caused by thermal and vibration fatigue cracking in areas of stress concentration. Failure of the spot-welds, which attach the heat transfer pins, can result in exhaust gas leakage. In addition to a carbon monoxide hazard, failure of heat exchanger surfaces can permit exhaust gases to be drawn into the engine induction system, causing engine overheating and power loss.
Exhaust Manifold and Stack Failures
Exhaust manifold and stack failures are usually fatigue failures at welded or clamped points (e.g., stack-to-flange, stack-to-manifold, and crossover pipe or muffler connections). Although these failures are primarily fire hazards, they also present carbon monoxide problems. Exhaust gases can enter the cabin via defective or inadequate seals at firewall openings, wing strut fittings, doors, and wing root openings.
Internal Muffler Failures
Internal failures (baffles, diffusers, etc.) can cause partial or complete engine power loss by restricting the flow of the exhaust gases. If pieces of the internal baffling break loose and partially or totally blocks the flow of exhaust gases, engine failure can occur. [Figure 2]
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| Figure 2. An example of internal muffler failure. Muffler failure can be caused by erosion and carbonization, which in turn can lead to breakage blocking exhaust flow |
As opposed to other failures, erosion and carburization caused by the extreme thermal conditions are the primary causes of internal failures. Engine backfiring and combustion of unburned fuel within the exhaust system are probable contributing factors. In addition, local hot-spot areas caused by uneven exhaust gas flow can result in burning, bulging, or rupture of the outer muffler wall.